Superconductor data storage device



March 10; 1970 s. LACROIX 3,500,344

SUPERCONDUCTOR DATA STORAGE DEVICE Filed June 27. 1966 5 Sheets-Sheet 1no.1 M

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March 10, 1970 S. LACROIX SUPERCONDUCTOR DATA STORAGE DEVICE 5Sheets-Sheet 5 Filed June 27. 1966 FIGJS h bh United States Patent3,500,344 SUPERCONDUCTOR DATA STORAGE DEVICE Serge Lacroix,Champigny-sur-Marne, France, assignor to Socit Industrielle Bull-GeneralElectric, Socite Anonyme, Paris, France Filed June 27, 1966, Ser. No.560,552 Claims priority, appliciitiori France, July 29, 1965,

rm. (:1. (inc 11/44 U.S. Cl. 340-1731 3 Claims ABSTRACT OF THEDISCLOSURE This invention relates to data storage devices in which thedata are written in the form of persistent currents in a superconductingfilm.

The invention concerns more particularly devices of this type whichcomprise storage elements disposed in matrix form and in which theselection of a storage element is effected by means of control currentsapplied coincidentally along the storage element.

When currents are applied to control conductors disposed along thesuperconducting film, induced currents of opposite direction are set upon the surface of the latter, and when the density of the currentsinduced in the superconducting film locally reaches a certain criticalvalue, a small zone of this film changes to the resistive state at theplace in question.

The formation of resistive zones results in a local dissipation ofenergy and a modification of the state of equilibrium of the currentsinduced in the superconducting film.

The characteristic phenomena of the writing and reading of data indevices of the type under consideration result from the creation of suchresistive zones in welldefined regions of the superconducting film, andit has been discovered that certain disturbances which are likely toresult in the loss of written data are due to the appearance ofresistive zones in certain other regions of the superconducting filmwhich are separate from the first. I

Now, the critical current density, i.e. the value of the current densityfrom which resistive zones are formed in the superconducting film at theoperating temperature of the device, depends upon the nature of thesuper conducting film and upon its thickness.

In known storage devices of the type under consideration, thesuperconducting film is of constant nature and of constant thickness.

A superconducting-film storage device according to the invention ischaracterised in that the nature and the thickness of thesuperconducting film, in a region of the latter in which a return to theresistive state would be likely to disturb the operation of the device,are so chosen that the critical current density in this region issufficiently high to avoid such a transition during the operation of thedevice.

In order that such a transition may be obviated, it is necessary for thecritical density in this region to be higher than the maximum value ofthe current density "ice reached in this region when control currentspass through the control conductors of the device.

In order to provide a superconducting film having, in the region underconsideration, a sufficiently high critical density and in particular ahigher critical density than the regions in which the formation ofresistive zones is necessary for the operation of the device, variousmeans may be employed, more particularly the formation on the regionunder consideration of an additional thickness of the same substance asthe remainder of the superconducting film, or of a different substance,or the local introduction of an appropriate substance with or withoutincreased thickness by diffusion or local insertion.

The invention may with advantage be applied to storage matricescomprising a continuous superconducting film which is common to all thestorage elements. In this application, it permits of limiting thereciprocal actions between neighbouring storage elements.

For a better understanding of the invention and to show how it may becarried into effect, the same will now be described, by way of example,with reference to the accompanying drawings, in which:

FIGURE 1 is a plan view of a persistent-current storage element of afirst type to which the present invention is applicable,

FIGURE 2 is a section along 2-2 of FIGURE 1,

FIGURES 3 to 10 are plan and sectional views of various storage elementsaccording to the invention of the same type as that illustrated inFIGURES 1 and 2,

FIGURES 11 and 12 are plan views of storage elements of a second type towhich the invention is applicable, 1

FIGURES 13 and 14 are plan views of storage elements according to theinvention of the same type as those illustrated in FIGURES l1 and 12,and

FIGURES 15 and 16 are plan views of storage matrices according to theinvention.

The storage element illustrated in FIGURES 1 and 2 comprises acontinuous sheet 10 of a superconducting substance having a weakcritical field, such as tin or indium. This sheet is referred to by theexpression superconducting film in the remainder of the description. Thestorage element comprises in addition control conductors l1 and 21 inribbon form which are disposed on one of the faces of thesuperconducting film 10. These conductors are made of a superconductingsubstance, such as lead, which has a relatively high critical field.

In FIGURE 1, the cross-hatched areas A1, A2, B, C1, C2, D1, D2 and D3represent zones of the superconducting film 10 which become resistive orwhich are capable of becoming resistive, during the operation of thestorage element.

A binary datum element is represented in the storage element in the formof persistent currents flowing in a particular direction through thesuperconducting'film 10 around the resistive zones A1 and A2, each ofwhich extends from one face of the superconducting film to the other,and through which there exists a magnetic flux due to the saidpersistent currents.

The reversal of the direction of the persistent currents for the purposeof substituting one of the binary digits for the other in the storageelement assumes the momentary appearance, between the resistive zones A1and A2, of an intermediate resistive zone B which merges the two zonesA1 and A2 into one.

During the operation of the storage element just described, regardlessof whether this is during the passage of a current of normal valuethrough each of the control conductors for controlling the storageelement, or during the passage of a current having this same normalvalue through only one of these control conductors, re-

sistive zones may appear inregions of the superconducting film which areseparate from those in which the zones A1, A2 and B are situated. Thisis the case, for example, with the zones C1, C2, D1 and D2- which aresituated along angles formed by the edges of the control conductors 11and 21. The appearance of such zones is likely to cause disturbances inthe operation of the storage element.

In the storage elements according to the invention, the superconductingfilm constituting the writing medium is made such that, in those regionsof this superconducting film whose return to the resistive state must beavoided during the operation of the storage element, the criticalcurrent density is higher than the maximum current density which can bereached in this region during the passage of a control current througheither one of the control conductors or through both simultaneously.

This result may be achieved by various means.

In the figures accompanying the present specification, the areas hatchedwith chain lines represent regions of the superconducting film in whichthe critical density satisfies the aforesaid condition.

FIGURES 3 to 10 illustrate, as non-limiting examples, variousconstructional forms of superconductor storage elements embodying thepresent invention.

The regions 101 of the superconducting film 10 of the storage elementillustrated in FIGURES 3 and 4 are thicker than the region 102 of thisfilm, so that the critical density is higher in the regions 101 than inthe region 102.

The critical density of the superconducting film in the region 101 isthus brought to a value higher than the maximum value which can bereached by the current density in this region, during the passage of acontrol current through either one of the control conductors 11 and 21or through both simultaneously.

The formation of resistive zones in the region 101 of thesuperconducting film 10 during the operation of the storage element isthus avoided.

The cross-hatched areas A1, A2 and B of FIGURE 3 then represent the onlyresistive zones which are formed in the superconducting film in thecourse of the operation of the storage element. These zones are situatedin the. region 102 of the superconducting film, and they are formed inthe same Way as in the storage element illustrated in FIGURE 1.

Although the additional thicknesses of the super-conducting film 10 ofthe storage element illustrated in FIGURES 3 and 4 are situated on thatface of the said film' which is on the same side as the controlconductors 11 and 21, it is to be noted that storage elements accordingto the invention may be provided by disposing these additionalthicknesses on the opposite face of the superconducting film.

'Inthe regions 101 of the superconduction film 10' of the storageelement illustrated in FIGURES 5 and 6, superconducting films 1010 aresuperimposed on the superconducting film 10. Any superconductingsubstance which is capable of satisfying the conditions for theconstruction of the storage element may be employed to form thesesuperimposed superconducting films 1010.

Although in the storage element illustrated in FIG- URES 5 and 6- thesuperimposed superconducting films 1010 are disposed-on that face of thesaid film which is on the same side as the control conductors 11 and 21,it is to be noted that storage elements according to the invention maybe produced by providing superimposed super-conducting films on theopposite face of the superconducting film.

The composition of the regions 101 of the superconducting film of thestorage element illustrated in FIG- URES 7 and 8 is different from thatof the region 102 of the said film. The composition of each of theseregions is so chosen that the critical density therein has the desiredvalue. This may be effected by local diffusion of appropr ate subs ancein the sup rcsnducting film.

In the storage element illustrated in FIGURES 9 and 10, thesuperconducting film 10 is formed of separate elements consisting ofdifferent substances and forming the regions 101 and 102 respectively.

In the foregoing, it has been explained with reference to FIGURE 3 to 10how the invention may be applied to storage elements of the typeillustrated in FIGURES 1 and 2, and various means which may be employedin this application to obtain the desired critical density in the chosenregions of the superconducting film have been indicated. It is obviousthat the invention is also applicable with advantage to storage elementsof diiferent types, for example to the storage elements illustrated inFIGURES 11 and 12. In the latter storage elements, the establishment ofthe persistent currents representing data necessitates the formation ofresistive zones A1, A2 and B in the superconducting film 10, whiledisturbances may result from the return of the zones D1 and D2 to theresistive state.

FIGURES 13 and 14 illustrate storage elements of the type illustrated inFIGURES l1 and 12 respectively, embodying the present invention.

In these storage elements, the superconducting film 10 is so constructedthat the critical current density in the regions 101 represented inFIGURES 13 and 14 by areas hatched with chain lines is higher than themaximum current density which can be reached in these regions during thepassage of a control current through either one of the controlconductors or through both simultaneously.

The various means indicated in the foregoing make it possible to providea superconducting film having such regions.

Storage elements having the features set forth in the foregoing may beemployed in the construction of the storage matrices according to theinvention.

FIGURES 15 and 16 illustrate by way of example storage matricesaccording to the invention in which the writing medium is asuperconducting film 10 common to all the storage elements, and in whichthe control conductors 11, 12, 21 and 22 have a configuration such thatthey provide with the superconducting film 10 storage elements accordingto the invention of one of the previously described types. The storageelements of the storage matrix illustrated in FIGURE 15 are of the typeillustrated in FIGURES 3 to 10, and those of the matrix illustrated inFIGURE 16 are of the type illustrated in FIGURE 13.

In these storage matrices, the superconducting fil-m 10 is soconstructed that the critical current density in the regions 101,represented in FIGURES 1S and 16 by areas hatched with chain lines, ishigher than the maximum current density which can be reached in theseregions during the operation of the said storage matrices.

In the superconducting film 10 of the storage matrix illustrated inFIGURE 15, the region 102 comprising the resistive zones A1, A2 and Bwhose formation permits the writing and reading of data in a storageelement of the matrix is entirely surrounded by a region 101 in whichthe critical density satisfies the aforesaid condition. It is possibleby this arrangement to prevent reciprocal actions between the storageelement under consideration and the neighbouring storage elements.

In addition, in the superconducting film 10 of the storage matrixillustrated in FIGURE 15, the edge 1020 of such a region 102 issufliciently far from the zones A1, A2 and B for the characteristicphenomena of the writing and reading of data in the storage elementunder consideration to depend only upon the properties of the substanceconstituting the superconducting film in the region 102 underconsideration and not upon the dimensions or the state of the edges ofthe said region. Thus, the small differences in shape and dimensionswhich may exist in the regions 102 of the superconducting film 10 haveno effect on the phenomena under consideration.

In. the supercoaduc ng fi m 10 of the sto ag ma ix illustrated in FIGURE16, the regions 102 corresponding to storage elements disposed on acommon diagonal of the matrix form one and the same region, so that theregions 101 and 102 of the superconducting film form parallel stripshaving readily obtainable constant widths.

It will be obvious that this arrangement may also be adopted for thestorage matrix illustrated in FIGURE 15.

I claim:

1. A data storage device including at least one storage element, saidelement comprisinga sheet of superconducting material, said sheet havinga first area and a second area; and two control conductors in the formof ribbon-like strips of superconducting material, said controlconductors having at least a portion of their length superimposed atdifferent levels above said first area} and being magnetically coupledto said first area; said first area having a characteristic criticalcurrent density which is lower than the maximum current density obtainedin said first area by coincident control currents flowing through saidcontrol conductors; and said second area having a characteristiccritical current density which is higher than that of said first areaand higher than the maximum current density obtained in said second areabysaid control currents, whereby resistive zones are created in thefirst area under the action of flow of current and no resistive zone iscreated in the second area.

2. A data storage device including at least one storage elernent, saidelement comprising a sheet of superconducting' material of constantcomposition, said sheet having a first area and a second area, thethickness of the sheet in said first area being smaller than thethickness of said sheet in said second area; and two control conductorsin the form of ribbon-like strips of superconducting material, saidcontrol conductors having at least a portion of their lengthsuperimposed at different levels above said first area and beingmagnetically coupled to said first area; said first area having acharacteristic critical current density which is lower than the maximumcurrent density obtained in said first area by coincident controlcurrents flowing through said control conductors, and said second areahaving a characteristic critical current density which ishigher than themaximum current density obtained in said second area by said controlcurrents, whereby resistive zones are created in the first area. underthe action of flow of current and no resistive zone is created in thesecond area.

3. A superconductor memory matrix comprising a sheet of superconductingmaterial of constant composition, said sheet having a plurality of firstand a plurality of second areas, and two sets of control conductors inthe form of ribbon-like strips of superconducting material, said twosets of control conductors 'being respectively associated with the rowsand the columns of the matrix, each control conductor of one set andeach control conductor' of the other set having a portion of theirlength superimposed at ditferent levels above one of said first areasand being magnetically coupled to said first area, the thickness of thesheet in each first area being smaller than the thickness of said sheetin each second area, each first area having a characteristic criticalcurrent density which is lower than the maximum current densityobtained, in said first area by coincident control currents flowingthrough the respective two control conductors associated with said firstarea, each second area having a characteristic critical current densitywhich is higher than the maximum current density obtained in said secondarea by said control currents, whereby resistive zones are created ineach first area under the action of currents through the respective twocontrol conductors associated respectively with each first area and noresistive zone is created in any one of said second areas.

References Cited UNITED STATES PATENTS 2,981,933 4/1961 Crowe et al.340173.1 X 3,172,086 3/1965 Wendt 340173.1 3,234,439 2/1966 Alphonse340173.1 X 3,283,282 11/1966 Rosenberg 340-173.1 X

BERNARD KONICK, Primary Examiner J. F. BREIMAYER, Assistant Examiner US.Cl. X.R. 307306

